Advertisement

Advertisement

Found: crater from Deep Impact’s comet crash

By David Shiga

NASA’s Stardust probe saw different sides of Comet Tempel 1 during its close encounter with the comet on 15 February 2011

(Image: NASA/JPL-Caltech/Cornell)

At right, a Stardust image shows a crater blasted out of Tempel 1 by NASA’s Deep Impact mission. Arrows point to the crater’s rim. At left is a composite image of the same area made by Deep Impact prior to the collision in 2005

(Image: NASA/JPL-Caltech/Cornell)

A NASA spacecraft whizzed by a comet on Tuesday and obtained the first images of a crater gouged out by a predecessor mission in 2005. The images will teach scientists about comet structure, but come more than five years later than they had once hoped, with the wait extended a few more agonising hours by a last-minute software glitch.

On 4 July 2005, NASA threw a 372-kilogram impactor into Comet Tempel 1 as part of its Deep Impact mission. A flyby spacecraft recorded images of the impact from a safe distance, but the cloud of impact debris and a flawed camera made it impossible to see the crater itself. Studying the crater could have revealed more about the interior composition and structure of comets.

Advertisement

Now, a second NASA spacecraft has flown by the comet, giving scientists a second chance at spotting the crater.

The Stardust spacecraft, used previously to collect material from Comet Wild 2, flew within 178 kilometres of Tempel 1 at 0439 GMT on Tuesday. It snapped 72 high-resolution images of the comet’s solid core, or nucleus.

Weak surface

The images revealed a 150-metre-wide crater at the Deep Impact collision point that was not present in 2005 (see image). The crater is a subtle feature in the images, but it appears consistently in multiple views from the spacecraft. “So I feel very confident that we did find the [impact] site,” said mission member Peter Schultz of Brown University in Providence, Rhode Island, at a press briefing on Tuesday.

The crater’s features are “subdued” rather than sharply defined, like those of craters made in hard materials like rock. “The message is&colon; This surface of the comet where we hit is very weak,” said Schultz.

The crater also has a small mound in its middle, indicating that some of the material thrown up by the impact was drawn by the comet’s gravity back down into the crater, he said&colon; “In a way, it partly buried itself.”

Three become one

Tempel 1 is the first comet to be imaged close up twice, with the comet making roughly one orbit of the sun in between the visits by the Deep Impact and Stardust spacecraft. That allowed scientists to observe changes to the comet’s nucleus that have occurred over time.

For example, a group of three pits seen on the comet in 2005 have since merged into one, apparently due to erosion of material from the comet in the interim, said the mission’s chief scientist, Joe Veverka of Cornell University in Ithaca, New York, at the press briefing.

A large, smooth patch on the nucleus appears to have shrunk as well, he said. “We have to spend time quantifying those changes and understanding what they mean,” he said.

Material shed by the comet gave Stardust a rough ride during the close encounter. It was pelted by about 5000 dust particles, along with about a dozen millimetre-sized bits of debris, though it came through unscathed.

‘Bursts and puffs’

The impacts came in bunches, suggesting the comet was throwing off material in a sporadic way, mission scientist Don Brownlee of the University of Washington, in Seattle, said at the briefing.

Comet Wild 2 behaved similarly when visited previously by Stardust. “They don’t just spew out things in a uniform way – they send it out in bursts and puffs,” Brownlee said.

Mission scientists were pleased with the trove of new data returned by Stardust, even though a glitch forced them to wait longer than expected to get the closest images of the comet.

Mission managers had commanded the spacecraft to send the images back out of sequence, with the closest images of the comet – showing the most detail – to be sent to Earth first. If the command had worked, the close-up images would have started arriving at around 0800 GMT on Tuesday.

But Stardust refused to cooperate, sending the pictures back in chronological order instead. Since each image takes about 15 minutes to download, that caused a long delay in the arrival of the most detailed ones. NASA delayed its press conference by about 3 hours as a result, in order to allow scientists more time to analyse the images.